1. Field of the Invention
The present invention generally relates to tuning-fork type vibratory gyros, and more particularly to a tuning-fork type vibratory gyro having a piezoelectric substance.
A gyroscope has been used to identify the current position of a vehicle such as an airplane, ship or a satellite. Recently, a gyroscope has been applied to devices for personal use, such as car navigation and vibration detection in video cameras and still cameras.
A conventional coma gyro detects an angular velocity by utilizing a principle in which a rotating a coma (disk) continues to rotate without any change of the attitude thereof while keeping the rotation axis even when a device equipped with the coma gyro is tilted. Recently, an optical type gyro and a piezoelectric type gyro has been developed and reduced to practical use. The principles of the piezoelectric type gyro was proposed around 1950. Various piezoelectric type gyros having, for example, a tuning-fork, a cylinder or a semi-spherical member have been proposed. Recently, a vibratory gyro having a piezoelectric member has been in practical use. Such a vibratory gyro has less measurement sensitivity and precision than those of the coma gyro and the optical gyro, but has advantages in terms of size, weight and cost.
2. Description of the Related Art
A description will now be given, with reference to FIGS. 1A and 1B, of a vibration of a tuning-fork vibrator.
A tuning-fork vibrator (hereinafter simply referred to as a tuning fork) 1 includes two arms 2 and 3, and a base 4 supporting the arms 2 and 3. Each of the arms 2 and 3 has a rectangularly shaped cross section. The tuning fork 1 has two different modes of vibrations, one of which is a plane-vertical vibration (FIG. 1A) and the other vibration is an in-plane vibration (FIG. 1B). Hereinafter, the plane-vertical vibration is referred to as an fy-mode vibration, and the in-plane vibration is referred to as an fx-mode vibration.
The fx-mode vibration and the fy-mode vibration are different vibration modes. As shown in FIG. 1A, the fy-mode vibration includes torsional vibrations (indicated by arrows depicted with broken lines), and hence has a steady point, indicated by a "x", which is located on the bottom surface of the base 4. This means that it is very difficult to support the tuning fork. The fx-mode vibration shown in FIG. 1B has a steady line (not point), which corresponds to the center line on the bottom surface of the base 4. Hence, it is possible to reduce the movement of the bottom surface of the base 4 to approximately zero by adjusting the length L of the base 4.
Generally, the fx-mode vibration is driven, while the fy-mode vibration is detected, so that an electric output signal due to Coriolis force can be obtained.
More particularly, as shown in FIGS. 2A and 2B, a holder 5 is elastically attached to the base 4 of the tuning fork 1. The fy-mode vibration gives torsional stresses to the holder 5 as shown by broken-line arrows in FIG. 2A, while the fx-mode vibration gives compressive stresses to the holder 5. By selecting an appropriate length L of the base 4 with respect to the total length of the tuning fork 1, it is possible to suppress most of the fx-mode vibrations around the base 4 and to substantially neglect the influence of the holder 5. Hereinafter, the combination of the base 4 and the holder 5 is referred to as a base portion.
On the other hand, a torsional displacement of the fy-mode vibration is very much greater than a longitudinal displacement of the fx-mode vibration. In the fy-mode vibration, a rotational motion (torsional displacement) about the center (the yz plane in the center of the holder 5) of the rotational motion in the holder 5 occurs, as shown in FIG. 2A. That is, in the fy-mode vibration, the tuning fork 1 and the supporting member vibrate integrally. Hence, it is possible to change the resonance frequency and/or the mechanical Q by changing the shape, material and weight of the holder 5 and to thus obtain the frequency band of the desired fy-mode vibration.
For example, if the weight of the holder 5 is changed by attaching a weight to the holder 5, the rotational motion of the holder 5 is suppressed. When the rotational motion of the holder 5 is suppressed, the detection frequency is decreased. If a weight of the holder 5 is added to the base 4, a similar effect will be obtained.
With the above in mind, the gyro shown in FIGS. 2A and 2B is supported by a board 6 such as a printed-circuit board, as shown in FIG. 3, which shows the gyro viewed from the bottom of the holder 5. More particularly, the arms 1 and 2 extend in the z-axis direction. A pin 8 penetrates through the center of the holder 5. The pin 8 is attached to the board 6 and is fixed thereto by a resilient member such as a resilient or elastic adhesive. In the structure shown in FIG. 3, the fx-mode vibration occurs in parallel with the plane of the board 6, while the fy-mode vibration occurs in a plane perpendicular to that of the board 6. Since the holder 5 is supported in the center thereof, the rotational motion of the holder 5 is not suppressed. Hence, it is easily possible to adjust the detection frequency by adding a weight to the holder 5 or the base 4.
However, the attachment structure shown in FIG. 3 has a disadvantage in that it is liable to be affected by an external vibration. If an external vibration is applied to the structure, the gyro will be swung about the attached position of the pin 8 in directions indicated by the two-headed arrow shown in FIG. 3. For example, a rotational motion different from that shown in FIG. 2A will be caused in the base portion by the external vibration, so that noise (particularly called cross-talk noise) will develop across the detection electrodes. Although unnecessary motions (which will occur in the x-, y- and z-axis directions) other than the motion shown in FIG. 3 will be caused, these motions will not greatly cause noise. That is, the unnecessary motion shown in FIG. 3 causes substantial noise. The cross-talk noise mainly caused by the unnecessary motion shown in FIG. 3 causes a detection error if the gyro shown in FIG. 3 is used in an environment in which a vibration always occurs, for example, if the gyro is mounted on a vehicle. The presence of such a detection error degrades the precision of detecting the angular velocity.
If the whole base portion is rigidly fixed to the board 6, the occurrence of a swinging motion of the gyro will be prevented. However, in this case, the base portion cannot be moved and it is no longer possible to perform the frequency adjustment by adding a weight to the base portion.